Production of cellulose acetate



Patented Aug. 24, 1937 UNlTE TATES PTENT OFFEQE Brown Company, Ber Maine No Drawing. Applic lin, N. H., a corporation of ation February 8, 1935,

Serial No. 5,580

'7 Claims.

This invention relates to the production of cellulose acetate. It deals with the use of Various kinds of base cellulose fibers for this purpose, including cellulose fibers of the nature of chemically liberated cellulose pulp and cotton and more especially the chemically liberated cellulose pulps derived from wood or other raw cellulosic materials that have been freed substantially completely from lignins, resins, pentosans, and other impurities by the chemical pulping of the raw cellulosic material and/or after-refining treatments of a physical and/ or chemical character.

In acetylating cellulose fiber used as the base or raw material, it is customary practice to steep the fiber in substantially dry condition in mixed acetylating acids containing strong mineral acid, such as sulphuric, to promote or catalyze the acetylating reaction into which the glacial acetic acid component of the mixture enters along with the cellulose. The fiber is mixed to form a thick fibrous. suspension in the acetylating reagent or mixture so as to minimize the amount of reagent tied up in the acetylation of a given amount of fiber and thus to minimize reagent losses as well as the expense of recovering the unconsumed Valuable acetic acid component from the used reagent.

In accordance with the present invention, the amount of mineral acid necessary to promote or 0 catalyze the acetylating reaction is reduced considerably by introducing or depositing substantially uniformly throughout the fiber preparatory to its acetylation a comparatively small amount of suitable mineral acid or acid salt. In other 35 Words, the cellulose fiber that goes into the acetylating reagent is, as previously, a substantially dehydrated fiber, but its individual fiber units have intimately associated therewith a quota of mineral acid capable of promoting or catalyzing 40 acetylation. Such preliminary conditioning of the fiber is advantageous in that not only does it make possible the use of less mineral acid in the acetylating mixture to conduce to the desired rate of acetylating reaction on the fiber, but it 45 tends to reduce degradation or hydrolysis of the cellulose in the course of the acetylating reaction and enables an easier and more economical recovery of the uncons-umed or residual acetic acid left in the excess acetylating reagent that is 50 extracted or separated from the acetylated fiber. In some instances, however, the introduction or deposition of mineral acid. throughout the fiber may be conducted with a view toward hydrolyzing or degrading the cellulose preparatory to an acet- 55 ylating or other esterifying reaction. Thus, the

mineral acid or acid salt may be added to the fiber in the form of an aqueous solution, whereupon the water may be evaporated by drying the solution-treated fiber under elevated temperature conditions conducive to the desired hydrolysis or degradation of the cellulose, which hydrolysis is attended by a lowering of the solution viscosity of the fiber and the realization of cellulose esters of lowered solution viscosity.

There are various ways of incorporating suitable mineral acids or acid salts throughout cellulose fiber preparatory to putting it through the acetylating reaction pursuant to the principles of the present invention. While it is possible to expose the fiber to a mist or fog of mineral acid, produced, for example, by emitting such acid or acid solution through an atomizer or spray nozzle, until substantially all the fiber units, whether present in porous sheet form or as a loose, voluminous mass of shredded or otherwise disassociated and substantially individualized fiber units, have absorbed substantially uniformly the desired amount of acid, it is preferable to treat the fiber with excess dilute solution of the mineral acid and then to get rid of the .water therefrom while retaining the appropriate small amount of mineral acid as a residue or deposit substantially uniformly on all its fiber units. If desired, highly volatile organic solvents, such as acetone, may be used alone or in admixture with water as the vehicle or carrier in which the mineral acid or acid salt is dissolved or suspended preparatory to the treatment of the fiber therewith. In such case, the volatile solvent may be one that can be evaporated quickly and at comparatively low temperature from the fiber so as to minimize hydrolysis or degradation of the cellulose during the drying of the fiber. In the case of bulk pulp, the pulp mass may be steeped in a dilute solution of the mineral acid, excess solution removed from the pulp as by pressing and centrifugating operations, and the residue Water content of the pulp substantially completely evaporated therefrom under temperature conditions permitting the retention of the mineral acid on the fiber without serious degradation or hydrolysis of the cellulose. Fiber in interielted or porous sheet form may be similarly treated, the sheeted fiber being, for example, dipped in the dilute mineral acid solution, pressed tree of excess solution, and then substantially completely dehydrated under suitable temperature conditions. Thus, dehydration of the fiber in either bulk or sheet form may be effected While avoiding serious mechanical or chemical degradation thereof by distilling or evaporating its aqueous content under a substantial vacuum, for instance, a vacuum corresponding to an absolute pressure of, say, 10 to 5 pounds or even less. If desired,

5 the evaporation of the water or other volatile carrier or solvent of the mineral acid may be promoted by causing warm air to circulate or flow past and into contact with the fiber as it is being dried under atmospheric pressure or under substantial vacuum. Thus, the fiber may be dried in a chamber equipped with suitable heating means and one or more suction or evacuating ports communicating with a vacuum pump and so arranged as to cause circulation or flow of the warm air being exhausted from the chamber past the fiber as it is being dehydrated. When using such strong, comparatively involatile and preferred mineral acids, as sulphuric and phosphoric, it is possible to dehydrate the fiber substantially completely, for instance, to a moisture content of about 1% to 3%, while retaining mineral acid as a residue distributed substantially uniformly therethrough.

When cellulose fiber is purified with an alkaline liquor and more especially an alkaline liquor of mercerizing activity preparatory to acetylation, it takes on what may be termed a passive state toward the acetylating reagent. This happens, for instance, when chemical wood pulp is purified with caustic soda solutions of, say, about 18% strength at room temperature. It has been found that cellulose fiber, such as mercerized fiber, displaying passivity toward an acetylating mixture may be restored to a more or less active state by associating with the fiber units such comparatively high or boiling point organic activating liquids as glycerine, ethylene glycol, lactic acid, which, particularly when added in controlled amount, do not deleteriously affect the quality of the resulting cellulose acetate. In accordance with the present invention, it is possible to deposit to good advantage on the cellulose fiber along with the mineral acid, as hereinbefore described, the desired percentage of acti- 45 vating agent, such as glycerine, ethylene glycol,

lactic acid, or their equivalents. Thus, when the mineral acid is applied in excess to the fiber either in bulk or sheet form as a dilute aqueous solution, it may be desirable to add to such dilute solu- 0 tion the desired percentage of anyone or more of the watersoluble activating liquids already mentioned, particularly when the fiber has acquired passivity toward acetylating mixtures through mercerization or otherwise.

The cellulose fiber conditioned for acetylation according to the present invention may be of widely variant composition. Generally speaking, however, it is not desirable to add to the fiber more than about 10% of mineral acid, based on 0 the dry weight of the fiber. Assuming that chemically purified wood pulp of high alpha cellulose content and/or cotton constitutes the raw material, the conditioned fiber may have a composition consisting essentially of:

5 Percent Cellulose fiber 96 Sulphuric acid 2 Water 2;

A similar kind of fiber may be conditioned to 0 have a composition consisting essentially of Per cent Cellulose fiber 94 Sulphuric acid 3 5 Phosphoric acid 3 Assuming that wood pulp or cotton has been purified with a mercerizing caustic soda solution for acetylation, the fiber may be so conditioned as to possess a composition consisting essentially The foregoing conditioned fibers or others of different compositions prepared in accordance with the present invention may be acetylated in the usual acetylating reagents, although, as already stated, they may be acetylated to advantage in acetylating mixtures of much lower than usual mineral acid content. Indeed, even taking into consideration the mineral acid bound up with the fiber as it is put into the acetylating re agent, it is possible to reduce markedly the total mineral acid content in the sphere of the acetylating reaction. The role of the mineral acid is evidently that of a catalyzer, abstracting from the sphere of reaction the water produced as one of the reaction products when chemical combination takes place between the cellulose and the glacial acetic acid of the acetylating mixture. The mineral acid may, however, play another or an additional role, for instance, producing an intermediate reaction product with the cellulose, which, in turn, may react with the acetylating mixture. Irrespective of the part played by the mineral acid, by initially combining with the cellulose fiber only a comparatively small amount of mineral acid as is done pursuant to the present invention, it is possible to realize a much more effective promotion or catalysis of the acetylating re action than when considerably more mineral acid exists in the body of the acetylating mixture surrounding the fiber, as in usual practice. In other" words, the present invention makes possible a localization of the mineral acid catalyst in higher concentration on or in the fiber structure than in the main body of the acetylating mixture, whereas, in the usual practice of acetylation, there is no such localization of mineral acid catalyst.

1. In the production of cellulose acetate from cellulose fiber of the nature of chemical wood pulp and cotton, those steps which comprise first treating such fiber substantially uniformly with a solution of sulphuric acid in a comparatively volatile solvent, substantially completely evaporating from the fiber its solvent content while retaining said sulphuric acid as a residual deposit thereon, and acetylating the resulting fiber in the presence of said residual sulphuric acid deposit.

2. In the production of cellulose acetate from cellulose fiber of the nature of chemical wood pulp and cotton, those steps which comprise first soaking the fiber with a solution of sulphuric acid in a comparatively volatile solvent, mechanically extracting excess solution from the soaked fiber, substantially completely evaporating from the fiber the solvent content of the remaining sulphuric acid solution while retaining its sulphuric acid content as a residual deposit on the fiber, and acetylating the resulting fiber in the presence of said residual sulphuric acid deposit.

3. In the production of cellulose acetate from cellulose fiber of the nature of chemical wood pulp and cotton, those steps which comprise first treating such fiber substantially uniformly with an aqueous solution of sulphuric acid, substantially completely evaporating from the fiber its aqueous content while retaining said sulphuric acid as a residual deposit thereon, and acetylating the resulting fiber in, the presence of said residual sulphuric acid deposit.

4. In the production of cellulose acetate from cellulose fiber of the nature of chemical wood pulp and cotton, those steps which comprise first treating such fiber substantially uniformly with a solution of sulphuric acid in a comparatively volatile organic solvent, substantially completely evaporating from the fiber its organic solvent content while retaining said sulphuric acid as a residual deposit thereon, and acetylating the resulting fiber in the presence of said residual sulphuric acid deposit.

5. In the production of cellulose acetate from cellulose fiber of the nature of chemical Wood pulp and cotton, those steps which comprise first treating such fiber substantially uniformly with a solution of sulphuric acid in a comparatively volatile solvent, substantially completely evaporating under vacuum substantially completely from the fiber its solvent content under temperature conditions such as to avoid serious hydrolysis of the cellulose while retaining said sulphuric acid as a residual deposit thereon, and

acetylating the resulting fiber in the presence of said residual sulphuric acid deposit.

6. In. the production of cellulose acetate from cellulose fiber of the nature of chemical wood pulp and cotton, those steps which comprise first treating such fiber substantially uniformly with an aqueous solution of sulphuric acid, evaporating under vacuum substantially completely from the fiber its aqueous content under temperature conditions such as to avoid serious hydrolysis of the cellulose while retaining said sulphuric acid as a residual deposit thereon, and acetylating the resulting fiber in the presence of said residual sulphuric acid deposit.

7. In the production of cellulose acetate from cellulose fiber of the nature of chemical Wood pulp and cotton, those steps which comprise first treating such fiber substantially uniformly with a solution of sulphuric acid in a relatively highly volatile organic solvent, such as acetone, evaporating substantially completely from the fiber its organic solvent content under temperature conditions such as to avoid serious hydrolysis of the cellulose while retaining said sulphuric acid as a residual deposit thereon, and acetylating the resulting fiber in the presence of said residual sulphuric acid deposit.

GEORGE A. RICHTER. 

